1. Description of the Prior Art
Residual petroleum oil fractions, such as the heavy fractions produced in atmospheric and vacuum crude distillation columns, are usually undesirable as feedstocks for most refining processes due to their high metals and sulfur content. The presence of high concentrations of sulfur and metals, the latter often being found in relatively large porphyrin molecules, precludes the effective use of residua as feedstocks for cracking, hydrocracking, and similar catalytic refining operations.
One method known for reducing the sulfur and metals content of residua is hydrodesulfurization, a process wherein a residuum, usually containing the bulk of the asphaltene components of the original crude from which the residuum was derived, is contacted with a catalyst, generally composed of cobalt and/or nickel components plus molybdenum components on a porous refractory oxide support, under conditions of elevated temperature and pressure and in the presence of hydrogen such that the sulfur components, are converted to hydrogen sulfide while the metals are simultaneously deposited on the catalyst.
It has been recognized that catalysts having specific pore size distributions and/or surface area characteristics are effective for hydrodesulfurization purposes. For example, U.S. Pat. Nos. 4,082,695 and 4,089,774 describe processes for removing metals and sulfur from residua with catalysts having specific pore size distributions.
Numerous processes have been suggested in the prior art to improve the porosity characteristics of support particles used in catalysts. One such process, disclosed in U.S. Pat. No. 2,890,162 involves impregnating a porous starting material with a molybdenum and/or cobalt metal component that serves as a pore size distribution growth promoter. Another such process, disclosed in U.S. Pat. No. 4,022,715, involves incorporating a blowing agent in the catalyst materials. Other processes provide only limited improvements in porosity before other desirable properties such as strength, attrition resistance, and the like are adversely affected.
In the present invention it has been discovered that lithium has unusual properties for modifying the pore size and/or surface area characteristics or porous refractory oxide particles. Accordingly, it is an object of this invention to provide a method using lithium promoters for increasing the average pore diameter and decreasing the surface area of porous refractory oxide particles, especially with respect to regenerated catalytic supports. A further object is to provide a method for substantially altering the physical properties of precalcined gamma alumina particles without causing substantial phase transformation to other forms of alumina. A further object still is to alter the physical properties of support particles to a greater degree than would be possible by calcining in the absence of lithium under more severe conditions than those employed with lithium promoters utilized in the invention. Yet another object is to provide catalytic supports having physical characteristics obtained through use of lithium promoters. Still another object is to utilize these catalytic supports as components of catalytic compositions, useful in the hydroprocessing of hydrocarbons, and most particularly in hydrodesulfurization. These and other objects and advantages of the invention will be apparent from the following description.